1. Field of the Invention
This disclosure relates generally to a method and an arrangement for detecting a leak in an anesthesia system.
2. Description of Related Art
Inhalation anesthesia is delivered using a re-breathing circuit comprising an inspiratory limb through which a patient gets an inspired breathing gas from a ventilator, an expiratory limb carrying an exhaled gas back to the ventilator, a Y-piece connecting the inspiratory and expiratory limbs to a further patient limb providing a gas communication pathway to patient lungs. An expired gas comprises a lot of expensive and environment-hostile anesthesia gases. The re-breathing circuit is used to return the expired gas to next inspiration. Before doing this, the gas must be cleared out from the carbon dioxide extracted from the patient lungs as waste product of a patient metabolism. The clearance is done in a CO2 absorber.
Traditional anesthesia ventilator comprises a bellows-in-bottle, which separates a ventilator drive gas from the circulating breathing gas. For the inspiration, outside of the bellows is pressurized with ventilator drive gas. This squeezes the bellows forcing the breathing gas within the bellows to flow towards the patient lungs. During the expiration, the drive gas pressure is released and the gas pressurized in the patient lungs flows out filling the bellows. Breathing gas is supplied to the re-breathing circuit as continuous flow using a fresh gas line. Once the bellows is filled, further gas flow to the circuit from the patient and the fresh gas line increases the circuit pressure that opens an over-pressure bleed valve for a removal of this further gas to a scavenging system.
An inspiration gas volume is controlled using a flow sensor measuring either the ventilator drive gas or the patient inspiration gas depending on the application. Measuring the drive gas excludes the fresh gas delivered to the circuit and the resulting breath volume would be larger by this fresh gas flow compared to the delivered drive gas volume. Measuring the patient inspiration gas flow includes both the ventilator drive gas induced flow from the bellows and the fresh gas flow. In some systems the ventilator drive gas is also compensated for the fresh gas flow in a way to reduce ventilator drive gas flow with increasing fresh gas flow.
The expiration gas volume is not controlled. The volume that is inspired is passively expired during following expiration. Some systems, however, are equipped to measure also expired gas flow for monitoring purposes. This is done with a flow sensor connected to the expiratory limb.
A breathing circuit tightness is essential for the successful patient ventilation. Leakages occur frequently because the breathing circuit is disassembled for cleaning and reassembled between patients. This assembly procedure may be complicated and is liable for errors causing leakages. Therefore, all anesthesia machines enclose a breathing circuit leak test that is performed before the patient is connected and the operation begins. However, circuit modifications, disconnections, and reconnections are needed every now and then for various reasons like a patient suction also during an operation. Also an endotrahceal tube sealing may loose, or inadvertent loosening of some tubing may occur in the busy and often tight anesthesia environment. These are not revealed by the pre-use leak test.
Anesthesia clinicians favor the bellows-based re-breathing circuit because the end-expiratory position of the bellows visualizes a circuit leak. Would the breathing gas leak out from the re-breathing circuit be larger than the compensatory fresh gas flow rate, the bellows does not rise to its full extent. As a leak indicator this is, however, not very sensitive, since the leak must be larger than the fresh gas flow rate, which may be anything from 0.5 to 6 L/min. Further disadvantage of the bellows-based system is that if the bellows gets empty due to insufficient leak to the fresh gas flow—ratio, there is no gas for the patient breathing and the patient ventilation stops.
In a new re-breathing circuit a drive gas—a breathing gas separation is done using a long gas channel where a gas column is moving back-and-forth during the course of the inspiration and the expiration. As an advantage, such system does not have any volume limitation and even in leak situations the patient breathing volume can be delivered. Also here, however, the leak visualization is lost.